Re: [process] Process.Spawn leaves the child in zombie state on posix

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2022-07-06 15:27:48

On 7/6/22 16:28, Benedek Tass via Boost wrote:
> Hi, everyone!
> 
> This bug was observed in 2022.07.05, using boost version 1.79.0.
> Similar behaviour was commented on before on online messageboards.
> 
> 
> Bug description
> 
> When starting a new, detached process with Process.Spawn in a posix system,
> if the parent process outlives the child, the child process remains in
> zombie state for the parent process' lifetime.
> The bug described above is demonstrated in the CMake project attached to
> this document.
> 
> 
> Analysis
> 
> The spawn function injects the syscall "signal(SIGCHLD, SIG_IGN)" with a
> functor of type boost::process::detail::posix::sig_init_. This is done to
> the forked child process, to no avail. And this is not done (and should not
> be done) in the parent process being too intrusive.
> 
> 
> Possible mitigation
> 
> Introducing an in-between forked process that serves as the parent of the
> to-be-spawned process with SIGCHLD set to SIG_IGN would prevent the spawned
> process to become zombie, and simultaneously does not disturb the parent
> process' signal handlers.
> Introducing another class alternative to sig_init_ with the functionality
> described above would be a reasonable approach. An implementation sketch of
> the double fork method can be found in the attached project.
> 
> 
> System used
> 
> OS: Ubuntu 18.04.6 LTS
> arch: x86_64
> compiler: gcc-7.5.0
> libc: libc-2.27
> boost: 1.79.0 (built from source)

You should probably report bugs on GitHub:

https://github.com/boostorg/process/issues

In the bug, it is always desirable to post a small compilable code
sample that reproduces the issue.

Regarding the proposed fix, it's not clear how introducing an
intermediate process would fix the parent not calling waitpid() or
equivalent. You'd just get a different zombie process.